The most elusive aspect of the study of enzyme action has been in quantitating those factors (part processes) which provide the driving force for catalysis. Thus, any study of these factors requires an accurate thermodynamic characterization of the monomolecular and bimolecular elementary steps in the catalytic sequence. Much of the research described here is involved with acquistition of thermodynamic (delta G , delta H, delta S) data on two elementary events in esterase catalysis, viz, formation of Michaelis complexes and covalent enzyme-substrate complexes. The ultimate goal is to use these data to seek quantitative information on some part processes believed to be important in enzyme action. The factors of interest include: (1) the role of enzyme in relieving steric strain by formation of acylenzymes from cyclic ester substrates (2) the nature of tetrahedral (hemiacetal) covalent complexes formed from specific aldehyde-enzyme interactions and (3) the evaluation of conformational and covalent bond rearrangement contributions to acylenzyme formation. The latter item involves the use of the random coil as a reference state for separation of the conformational and covalent bond rearrangement contribution.